The invention concerns a drive circuit, and in particular a varactor line drive circuit for integration into a synthesizer circuit.
The use of varactor tuning in frequency synthesizers is well known and conventionally requires that the varactor (a varicap diode exhibiting a capacitance inversely proportional to the magnitude of the reverse-bias voltage across it) be driven by a synthesizer stage via a drive transistor external to that stage. A typical scheme is shown in FIG. 1, in which a synthesizer stage 20 drives the varactor input 31 of a tuner stage 30 by means of an external drive transistor 32 in conjunction with a load resistor 33 coupled to a high-voltage supply 34. Instructions are passed from a control microprocessor 40 along an PC bus 41 to the synthesizer 20 to select the desired channel frequency in the tuner 30. These instructions are compared with the actual frequency of the tuner oscillator 33, which is conveyed along a line 38 to the synthesizer stage 20, to provide an error signal in the synthesizer stage. This error signal is then used to bring the frequency of the oscillator 33 nearer to the required frequency by arranging for it to vary the signal on the base of the drive transistor 32. The resulting variation in the collector voltage of transistor 32 causes the reverse bias on the varicap 34 to change, and it is this change in reverse bias which brings about a corresponding change in the capacitance of the varicap 34, thereby adjusting the frequency of the oscillator 33 to the required frequency. Finally, the correctly adjusted oscillator signal in the tuner 30 is mixed with an incoming RF signal from an antenna 35 in a mixer 36 to produce an output IF signal on a line 37.
It is a common requirement in the art to be able to integrate as many functions as possible onto one chip. However, up till now, considerable problems have beset the designer who wished to integrate a drive transistor such as that shown as transistor 32 in FIG. 1 into an integrated stage such as the synthesizer stage 20 shown in the same figure. This is because the varactor drive circuit is required to operate at anywhere up to 35 volts, although the breakdown voltages inherent in the manufacturing process used for the synthesizer stage 20 are only in the region of 10-20 volts, depending on which junction in the produced chip is being used as a reference.